Question

I have a data frame with a grouping variable ‘ID’ and some values (‘Value’):

dt <- data.frame(
        ID = c('A1','A2','A4','A2','A1','A4','A3','A2','A1','A3'),
        Value = c(4,3,1,3,4,6,6,1,8,4)
    )
dt
#    ID Value
# 1  A1     4
# 2  A2     3
# 3  A4     1
# 4  A2     3
# 5  A1     4
# 6  A4     6
# 7  A3     6
# 8  A2     1
# 9  A1     8
# 10 A3     4

I can calculate an overall rank order of the ‘Value’ column like this:

dt$Order <- rank(dt$Value, ties.method = "first")
dt
#    ID Value Order
# 1  A1     4     5
# 2  A2     3     3
# 3  A4     1     1
# 4  A2     3     4
# 5  A1     4     6
# 6  A4     6     8
# 7  A3     6     9
# 8  A2     1     2
# 9  A1     8    10
# 10 A3     4     7

But how can I calculate rank order within each ‘ID’, instead of a global rank order?

#    ID Value  rnk
# 1  A1     4    1
# 2  A2     3    2
# 3  A4     1    1
# 4  A2     3    3
# 5  A1     4    2
# 6  A4     6    2
# 7  A3     6    2
# 8  A2     1    1
# 9  A1     8    3
# 10 A3     4    1

In T-SQL, we can get this done as the following syntax:

RANK() OVER ( [ < partition_by_clause > ] < order_by_clause > )

Any idea?

Answer 1

My way but there’s likely better. Never used rank, din’t even know about it. Thanks, may be useful.

#Your Data
dt <- data.frame(
    ID = c('A1','A2','A4','A2','A1','A4','A3','A2','A1','A3'),
    Value = c(4,3,1,3,4,6,6,1,8,4)
)
dt$Order <- rank(dt$Value,ties.method= "first")

#My approach
dt$id <- 1:nrow(dt) #needed for ordering and putting things back together
dt <- dt[order(dt$ID),]
dt$Order.by.group <- unlist(with(dt, tapply(Value, ID, function(x) rank(x, 
    ties.method = "first"))))
dt[order(dt$id), -4]

Yields:

   ID Value Order Order.by.group
1  A1     4     5              1
2  A2     3     3              2
3  A4     1     1              1
4  A2     3     4              3
5  A1     4     6              2
6  A4     6     8              2
7  A3     6     9              2
8  A2     1     2              1
9  A1     8    10              3
10 A3     4     7              1

EDIT:

If you don’t care about preserving the original order of the data then this works with less code:

dt <- dt[order(dt$ID),]
dt$Order.by.group <- unlist(with(dt, tapply(Value, ID, function(x) rank(x, 
   ties.method= "first"))))

   ID Value Order.by.group
1  A1     4              1
5  A1     4              2
9  A1     8              3
2  A2     3              2
4  A2     3              3
8  A2     1              1
7  A3     6              2
10 A3     4              1
3  A4     1              1
6  A4     6              2